Its very common for fertile eggs to look okay and go for the full incubation time and then die. There are multiple causes, 1.The most common, is too much contact with moisture. The eggs should grow slightly and remain firm during the incubation. if they grow too much it means that they are absorbing a lot of water and the embryo's will be at risk of drowning when it comes time to hatch. Interestingly enough, when large water soaked eggs do hatch, the babies are usually smaller and weaker then eggs that showed no growth at all.
2., improper humidity,you can tell when its too low, the eggs will become indented. The higher the humidity is the better. moisture however is deadly, no water dropplets should come in contact with the eggs and you should be careful not to let the substrate become to damp from condensation. Misting the eggs is a bad idea.
3. fluctuating temperatures, this can kill the eggs but is probably the easiest thing to control, use a thermostat, 83-84 is the ideal hatching temperature.